In elevator installations, many automatic sliding doors are equipped with safety systems designed to detect potential interference with the closing operation of the doors. Such safety systems typically include a plurality of signal emitter sources disposed on one door, and a plurality of signal receiver sources disposed on the other door. The signal emitters emit a curtain of signals across the threshold of the elevator door which are received by the signal receivers. When the curtain of signals is interrupted, the safety system communicates with a door controller in order to either stop the door closing operation and open the doors, or to maintain the doors in an opened position, depending on the current door position.
A doorway safety system is described in U.S. Pat. No. 4,029,176 (Mills) that utilizes acoustic wave transmitters and receivers to detect objects or persons within an area near the elevator doors. The system detects objects positioned between the doors and across the threshold, and extends the zone of detection into the entryway. The transmitters send out a signal at an angle into the entryway. When an obstruction enters the detection zone, the signal reflects from the obstruction and is detected by the receivers.
Another doorway safety system described in U.S. Pat. No, 5,886,307 (Full et al.) discloses a three-dimensional system for detecting objects across the threshold and in the entryway. The system projects a curtain of light beams across the threshold and illuminates the area directly in front of the entryway with three-dimensional detection beams. The system detects obstructions between the doors and across the threshold if an obstruction breaks one or more of the beams. In addition, if energy from the three-dimensional beams reflects off of an object in the entryway into the three-dimensional receivers, the obstruction is also detected.
The above-described system for three-dimensional detection has significant shortcomings. For example, while curtain-type detection systems require a "break" in one or more curtain beams to indicate an obstruction across a threshold, three-dimensional detection requires a "connect" to indicate an obstruction. This inverted logic for three-dimensional detection results in a sensitivity to external sources of energy that are not problematic in single-plane or curtain-type detection systems.
Systems that use light as the energy source for three-dimensional detection are subject to interference from a variety of external sources. For example, sources of light external to the detection system located near the elevator installation can inadvertently be picked up by the light sensors of the detection system. If the light from an external source is modulated similarly to that transmitted by the door safety system, it can be picked up and interpreted as indicative of an obstruction. Such external sources of light may include fluorescent lighting systems, emergency strobe lights, and emergency vehicle beacons.
External sources of impulse-type, electrical noise may also result in inadvertent obstruction signaling in three-dimensional door safety systems. Sources of this type of electrical noise include relay type elevator controllers, as well as electromechanical door operators.